Recently, with the progress in the electronic technology, the tendency is towards a higher performance, smaller size and portability of electronic equipment. In keeping up therewith, there is an increasing demand for a high energy density secondary cell employed for these electronic equipment. The secondary cells hitherto employed for these electronic equipment include nickel cadmium cells and lead cells. However, these cells are low in discharge potential and leave much to be desired in obtaining high energy density cells.
Recently, studies and investigations in a non-aqueous liquid electrolyte secondary cell employing a material capable of being doped with and releasing lithium ions, such as lithium, lithium alloys or a carbon material, and a lithium composite oxide, such as a lithium/cobalt composite oxide, as a negative electrode and as a positive electrode, respectively, are under way. These cells are excellent in cyclic properties and have high cell voltage and high energy density, while being subject to less self-discharge.
Meanwhile, in the case of a hermetically sealed cell, if the internal pressure in the cell is raised for some reason,the cell is abruptly/destroyed and either lose its function as the cell or cause damage to peripheral equipment. On the other hand, if the above-mentioned non-aqueous liquid electrolyte secondary cell is of the hermetically sealed structure, and the electric current flows in more than a predetermined quantity during charging, the cell voltage is increased to cause decomposition of the electrolyte and consequent gas evolution to raise the internal pressure in the cell. Besides, should the supercharged state persist, abnormal reactions, such as abrupt decomposition of the electrolyte or the active material, are occasionally produced to raise the cell temperature promptly. For combatting these disadvantages, an explosion-proof cell of the hermetically sealed structure has been proposed. This explosion-proof hermetically sealed cell has a current-interrupting device which is operated in response to increased internal pressure of the cell, as shown herein in FIG. 1.
With this cell having the current-interrupting device, if the supercharged state progresses to cause gas evolution due to chemical changes in the cell and consequent increase in the internal pressure in the cell, the current breaking device comes into operation with the rise in the internal pressure to interrupt the charging current. AS a result thereof, the progress of the abnormal reactions in the cell may be terminated to prevent an abrupt increase in the cell temperature of an increase in the internal pressure in the cell.
However, it has been found that, if a non-aqueous electrode secondary cell of the above-described explosion-proof type having a hermetically sealed structure is produced by using a material capable of being doped with or releasing lithium ions, such as lithium, lithium alloys or a carbon material, and a composite lithium oxide, such as a composite lithium/cobalt oxide, as a negative electrode and a positive electrode of the cell, respectively, and the cell is brought to a supercharged state, the cell occasionally undergoes abrupt heat evolution with an abrupt increase in temperature or a premature destruction.
As a result of our researches into the causes of the heat evolution with rapid temperature increase of the cell due to supercharging or premature destruction of the cell, the present inventors have found that heat evolution with a rapid increase in temperature and a premature destruction of the cell are produced before the internal pressure in the cell in increased appreciably. It is an object of the present invention to provide a non-aqueous liquid electrolyte secondary cell provided with a current interrupting device in which, even when the cell is supercharged, the current interrupting device comes positively into operation to prevent heat evolution with the abrupt increase in temperature or the premature destruction of the cell.